Use PEP 570 syntax in third party stubs (#11554)

stubs/protobuf/google/protobuf/reflection.pyi

@@ -1,6 +1,6 @@
 class GeneratedProtocolMessageType(type):
     def __new__(cls, name, bases, dictionary): ...
-    def __init__(__self, name, bases, dictionary) -> None: ...
+    def __init__(self, /, name, bases, dictionary) -> None: ...
 
 def ParseMessage(descriptor, byte_str): ...
 def MakeClass(descriptor): ...